mozilla-js-to-source.js

function foo() {
  console.log('foo');
}

var bar = function () {
  console.log('bar');
};

console.log(foo.toSource());
console.log(bar.toSource());

console.log(foo.toString());
console.log(bar.toString());

result

function foo() {console.log("foo");}
(function () {console.log("bar");})
function foo() {
  console.log("foo");
}
function () {
  console.log("bar");
}